The Mineral anhydrite

Anhydrite is not a common mineral, as it easily alters to the much more common mineral Gypsum from the addition of water into its chemical structure. Anhydrite and Gypsum are chemically almost the same mineral, except Gypsum has the addition of water. In fact, the name of Anhydrite is derived from "An" and "Hydra" - meaning "without water" - in reference to its similarity to Gypsum but the fact that it lacks water. Some specimens only partly alter to Gypsum, leaving one part Anhydrite and the other part Gypsum. Many deposits that once contained much Anhydrite now contain an abundance of Gypsum which was formed by the alteration of the Anhydrite.

Anhydrite also exists as a relic of the past in several traprock occurrences, where the Anydrite dissolves and leaves a hollow cast around its original form. Epimorphs of Quartz and Prehnite over Anhydrite frequently form at certain localities, with the original Anhydrite totally replaced or dissolved.

Anhydrite sometimes occurs in arid regions, forming from the dehydration of Gypsum. Fine but usually small crystals may come from the rock area above salt domes, where the domes absorb all underground water and prevent it from entering the structure of the Anhydrite, which would otherwise cause it to alter to Gypsum. Anhydrite specimens in a collection may also alter to Gypsum if kept in moist conditions over a prolonged period of time. It is recommended that this mineral be stored in a dry area or with silica gel. Some specimens sold to collectors from old collections may actually be Gypsum that has altered due to improper care.

European occurrences include Altaussee, Styria, Austria; Leopoldshall, Stassfurt, Germany; and the Campiano Mine in Montieri, Tuscany, Italy. An interesting and famous locality of Anhydrite is the Simpleton Tunnel in Wallis, Switzerland, where it was found in white and lilac crystals during the construction of a railroad tunnel.

At Rio Grande Do Sul, Brazil, very large Quartzpseudomorphs after Anhydrite were found in long and flattened crystals. Some of the best Anhydrite on the mineral market is the light blue dense fibrousveins, some quite large in size, from the mines at Naica, Chihuahua, Mexico. In Canada, Anhydrite comes from Faraday in the Bancroft District, Hastings Co., Ontario.

In the U.S., Quartz, Amethyst and Prehnitepseudomorphs, as well as empty Anydrite casts, are plentiful at Paterson and Prospect Park, Passaic Co., New Jersey. Intact specimens, partially altered to white Gypsum, have also been found nearby in the Sam Braen Quarry in Haledon, Passaic Co., New Jersey. Other Anhydrite occurrences are Balmat, Essex Co., New York; Isle Royale, Houghton Co., Michigan; Ajo, Pima Co., Arizona; Bisbee, Cochise Co., Arizona; the Carlsbad District of Eddy Co., New Mexico; Death Valley, Inyo Co., California; and the salt domes of southern Louisiana.

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